Method of treating mica



Patented Dec. 22, 1942 2,306,292 METHOD or resume MICA Francis Clarke Atwood, Newton, Mass., assignor to Atlantic Research Associates, Inc., Newtonville, Mass., a corporation of Delaware No Drawing. Application December 1, 1939,

' Serial No. 307,183

1 Claim.

This invention relates to a modified mica, and to a method of treating mica.

One object of the invention is to provide a novel and improved method of treating mica, to delaminate the same in a superior, economical and highly eflicient manner.

It is an object of this invention to provide a finely comminuted mica, the particle formation thereof being characterized by its extreme thinness and its retention of a laminar structure.

A further object of the invention is to provide a novel method of wet grinding mica, by which the same may be ground into particles of extremely small size having: a preferred thinness ratio thereby renderinggthe same suitable for many industrial and commercial purposes.

A further object of my invention is to provide mica particles, which in addition to the thinness and flake-like structure mentioned heretofore, have a surface contour facilitating the orientation of the mica particles in films and coating compositions in which the mica functions as a pigment.

A still further object of the invention is to produce a modified mica, having its sm'face characteristics altered to render it more suitable for use for various industrial purposes, such as for example in the production of plastic or fluid compositions for coating, painting, and similar purposes in which the modified mica is incorporated in a vehicle.

With these general objects in view, and such others as may hereinafter appear, the invention consists in. the novel modified mica, and in the novel method of treating the same to effect delamination thereof, and particularly in the novel method of wet grinding mica, hereinafter described and particularly defined in the claim at the end of this specification.

Other objects and advantages of the invention will be apparent to those familiar with the art from the following description.

Prior to the treatment in accordance with the present invention, the mica may be cleaned in the manner described in my Patent No. 2,177,358.

The method of the present invention contemplates the delamination of the mica bodies or sheets by treating the mica with a deflocculating agent, an organic wetting agent or other delaminating assistant or surface modifying agent, in the presence of water or other suitable lubricant. Upon working of the mica sheets while immersed in the lubricant and the deflocculating or wetting agent, the liquid works in between the lamliquors.

thin mica layers by destroying the natural adhesion of the mica crystals which bind the lam inae into the composite sheet, thus permitting.

; culating agents such as sulfite waste liquors and modifications thereof, glucose, lignin, and tannin It has been found that solutions containing polyoses also comprise satisfactory deflocculating agents, examples thereof including glucose, pentose, arabinose, dextrose and any other polyoses. While I prefer the materials above mentioned, I may use other known deflocculating agents, such as the soluble silicates. The said deflocculating agents, in addition to facilitating delamination' of the mica sheets,

also serve to prevent flocking of the delaminated mica into undesirable agglomerates which resist suitable dispersion. The amount of the deflocculating agent is not critical and very small amounts accomplish the desired result, for example, as little as M; to V percent (based on the weight of the mica) depending on the emciency of the agent used. A larger amount can be used, but is unnecessary.

Proper delamination of the mica sheets is of importance in obtaining the physical requirements of the mica particle structure, which permits of the novel and diversified uses to which the mica product of this invention may be adapted. Treatment of the raw mica in accordance with the method of the invention results in particles, at least 90% of which have a thickness of not more than 2 microns. This high degree of delamination is effected by flexing or working the mica sheets in the presence of a lubricant, such as water or other liquid in combination with one of the aforedescribed deflocculating agents. The flexing of the laminar structure of the sheet serves to open minute spaces between the laminae which permit the entry of the lubricant and deflocculating agents therein inae of the mica to facilitate separation of the 55 to weaken the crystalline adhesion between the laminae and allow separation of the laminar sheets of extreme thinness.

The flexing of the mica in the presence of the lubricant and the deflocculating agent may be carried out as a separate operation, followed by agrinding operation if desired, or the delaminating treatment may occur during wet grinding of the mica, the flexing and working of the mica necessary to effect the desired delamination being incident to the grinding operation.

' ing the adhesion therebetween.

when the delamination is carried out simultaneously with the grinding of the mica, a deflocculating agent of the character specified may be employed in the water or other liquid used in a wet grinding operation, and there results individual particles or flakes of the desired thinness as well as an extremely finely ground prodnot. The deflocculating agent also facilitates the grinding operation and thus enables substantial savings in time and power to be obtained thus producing much greater capacity with given apparatus. The exact mechanical details of the I grinding are not critical except that mills are preferred which give a mulling action without a shattering or crushing action. The grinding action preferred is that which will permit the flakes to slip apart with a minimum amount of fracture. The grinding may be accomplished by the usual mulling methods such as with a pan mill. The grinding may be continued until the mica particles have the desired size and thinness, or

the ground product may be screened, for example through a screen such that 90% of the particles will be finer than 325 mesh. A closed cycle grinding operation may also be employed in which a certain amount of the product of the desired size is continuously removed from the grinding zone, and the remainder ground further.

While the mica is cleaned as far as possible before being treated in the manner described herein, there are usually small amounts of clay and similar impurities adhering to or present between the laminae. The defiocculating agent has the additional function of deflocculating such impurities during the delamination or grinding so that they remain in suspension at the'time the mica is removed from the lubricant, thus insuring a mica product free from such impurities.

By means of my invention, an extreme thinness. The flakes, therefore, can be designated as at least 90% having a diameter-thinness ratio greater than 10, and in many instances as high thinner at their as 30 or above.

The particles obtained inuthis way by wet grinding with a deflocculating agent as herein described, besides the above referred to proper- ,ties, have a very desirable surface contour in that the surfaces converge to a great thinness at the edges of the flakes, that is, the flakes are edges than at their center portion, thereby resulting in faces approaching a convex formation. A

It has been'found that a flakeexhibiting the above properties possesses superior orienting abilities readily assuming, a flat position when applied to a surface. When incorporated into paints and coating'materials the flakes produce .superior films which possess greater covering ness in the mica flakes, namely at least 90%.,

having a maximum thickness of 2 microns, can be obtained upon wet grinding or the mica, although the flake is characterized by a retention of its essential flake or laminar structure, as contrasted with the relatively thick flakes of onehalf thousandths of an inch (12 microns) and the ground'mica dust previously known. The particles. because of their thinness and laminar structure provide a flake having a smaller thickness than length or diameter. A large portion of the mica flakes have a thickness of less than 1 micron and at least about 90% are under 2 microns in thickness; in general they may be saidto have an average thickness of less than one micron. Ninety percent of the particles pass a 325 mesh screen and the diameter of the average particle is about 10 to 15 microns. In general, the width or diameter of the flakes is approximately 10 to 30 times greater than their thickpower than other types of ground mica of which I am aware. The filming or orienting property of the mica ground in accordance with the present invention approaches, and in some respects may be regarded as superior to the orienting of flake aluminum and other metals.

The mica produced in accordance with my invention is to be distinguished from the relatively large reflecting mica flakes such as used in ornamenting wall paper. The mica I produce may be used as a. pigment in a paint film without imparting any substantial reflecting or sparkling appearance to the film and to produce a smooth film of enhanced hiding power. Mica flakes which have a relatively great thickness, such as those known heretofore, cannot be overlapped in a film so as to lie fiat in substantially the same plane, but ratherform raised portions at the pointsof overlap with the planes of the thick flakes at an angle to the surface of the film or surface to be coated. In accordance with my invention, because of the thinness of the flakes, they can be placed in overlapping fashion and yet remain substantially parallel to the surface. The converging character of the flakes at their ends makes it possible for the flakes to overlap with ease as distinguished from thicker flakes of a great many laminae which have a tendency to become dog-eared at their edges, thus preventing the desired overlap.

After wet grinding of the mica, the flakes may be separated from the liquid lubricant by a centrifug'al operation as described in my application Serial No. 154,133, filed July 17, 1937, now Patent 2,204,063. The mica flakes produced in accordance with my invention have noticeably different and superior properties from mica produced by prior art processes, particularly that resulting from ordinary wet grinding methods. It is believed that the deflocculating agent or organic wetting agent remains in small amounts upon the individual particles of mica to form a thin film thereon since the flakes are not washed prior to their separation from the lubricant. In some cases it appears that the surface of the mica particle is actually modified; whether this is the result of hydration, neutralizaiton or some other means such as increased polarity, is not exactly known, but there may be this action in addition to the molecular fllm of wetting agent left on the surface. The film or sheath on the particles imparts improved dispersing and mixing qualities to the product, and provides an unusually satisfactory base for receiving a color pigment or similar agent in admixture with the mica particles. In this altered capacity the mica particles act as effective pigment dispersing agents when incorporated in a painting composition, small particles of the pigment adhering to the convex surfaces of the coated mica flakes.

The characteristics above described contributing to the success with which the flakes may be incorporated into various vehicles such as paint and plastic vehicles are apparent from the pH value of the mica. It has been observed that the completeness and permanence of the suspension of the mica particles in the said vehicles corresponds to the pH value of the mica flakes. Particles of natural mica, and particularly with mica of the muscovite type, are characterized by a pH value usually ranging in the neighborhood of 5.5-5.8. The mica particles resulting from my process have a different pH value which is more efiicient in obtaining a dispersion and suspension in a paint vehicle. The difference in pH value of the flakes is attributed to the incorporation of the deflocculating or wetting agents of the type above referred to in between the laminae of the mica, and by the coating on the mica of the deflocculating or wetting agent. This procedure results in a change of the pH value of the mica particles from their natural range to a range of from 7 to 9, whereby more satisfactory dispersion and permanent suspension of the particles is produced.

Measurements of pH value are commonly thought of as designations of hydrogen ion concentration of a solution. However, many solids manifest a surface phenomena which is commonly designated in terms of pH. This pH value is particularly significant in finely divided pigments and determines to a great extent the dispersing qualities of a pigment. The pH value of the mica resulting from my invention is such as to have the optimum dispersing qualities and is entirely different in this respect from ordinary ground mica.

The pH of the mica particles may be ascertained by any suitably accurate method for determining the pH of solids. A preferred method comprises mixing,with the mica flakes the minimum amount of distilled water necessary to dampen them and measuring the pH of the dampened particles by inserting the glass electrodes of a conventional pH meter into the mass of dampened particles. The'pH value is read on the meter in the usual way. Another method of determining pH values of pigments is described in Circular No. 492 of the National Paint, Varnish 8: Lacquer Association, Inc., issued October, 1935. It may also be determined colormetrically by inethods well known and described in the literaure.

The mica produced in accordance with my in- .vention does 'not exist in nature and is to be distinguished in size, particle shape, and physical properties in paint films from that produced by ordinary grinding methods.

The finely comminuted mica product obtained herein with its novel flake structure makes its use as a pigment base in painting compositions, or similar coating compositions, particularly desirable. For instance, the product of the invention may be incorporated with conventional pigments or metallic pigments to form a composite pigment having greatly improved filming, working, and covering characteristics, the paint film being of markedly increased durability and weather resistance. Because of the superior filming and orienting properties of the mica product of'the invention, a coating composition embodying the mica exhibits covering properties which in many respects are superior to the high covering values obtained with flake aluminum andother metallic 'fiake paint pigment. The pigment may be used in oil paints or in water paints such as the casein paste type. r

The word mica as used in this description and in the following claim refers only to the material ordinarily referred to by that name, i. e., an elastic, laminated, silica mineral. While the invention has been described with reference to certain preferred embodiments, the use of equivalents where possible is contemplated as being within the scope of the invention, which is, to be limited only as set forth in the appended claim- This application contains subject matter in.

common with application Serial No. 90,959, filed July 16, 1936, and additional subject matter not contained in said application, and was filed while a renewal of said application was pending.

I claim:

The method of wet grinding mica, which consists in grinding the. mica (1) by flexing and working the mica sheets and simultaneously (2) by reducing the size of the mica laminations, both of said operations being carried out in the presence of water containing a mica defloceulating agent selected from the group consisting of a polyose, sulphite waste liquor and modifications thereof, lignin and tannin liquors, and soluble silicates, whereby to efi'ect the introduction of the water and defiocculating agent in between the laminae of the mica during the flexing and working of the laminar structure and the reduction in the size of the laminations to form fiat flake-like mica particles of small size and extreme thinness suitable for use as a pigment.

FRANCIS CLARKE ATWOOD. 

